1. Field of the Invention:
The present invention relates to the field of magnetic memory systems for the storage of binary digital data, and in particular to a random access memory device which utilizes stable ferromagnetic domain states for binary data storage.
2. Description of the Prior Art:
A serial access memory system utilizing stable ferromagnetic domain states for binary data storage was proposed by L. J. Schwee in the publication "Proposal on Cross-tie Wall and Bloch-line Propagation in Thin Magnetic Films" IEEE Transactions on Magnetics, IMAG 8, No. 3, pages 405 through 407, September 1972. In such a memory system, a thin film of ferromagnetic material of approximately 81% nickel and 19% iron is initially magnetized and thereafter stable domain states which constitute two stable digital storage states can be established by the appropriate application of magnetic fields to create in one case a stable Neel wall state, and a second state in which a Cross-Tie Block line domain wall is created. The digital data can be written into the memory by application of an appropriate magnetic field and the digital data can be read out using a magneto-resistive effect.
U.S. Pat. No. 3,868,660 teaches the significance of the shaping of the thin film of Permalloy magnetic material with spaced serrated edges being provided for the strips of film of magnetic material. U.S. Pat. No. 4,473,893 teaches a cross-tie random access memory system with an X-Y array of discrete memory elements formed of serrated Permalloy film and having aligned rows of addressing conductors superposed over the discrete memory elements in the X row, and serpentine-shaped Y column conductors aligned over and intersecting with the X row conductors over the individual memory elements of the X-Y memory array. In this prior art cross-tie memory, the digital data is written onto the memory elements using half address signals applied to both the row and column conductors. This system also includes a row of reference memory elements which are utilized during the read-out operation with a differential read-out being had between the memory elements with the stored digital data and the reference memory elements.
The cross-tie random access memory technology offers the following desirable characteristics: non-volatility, non-destructive read-out, radiation hardness, high density, and a large temperature operating range. The read-out of digital data from such cross-tie random access memories utilizes the magneto-resistive effect in which upon application of a magnetic field to the memory element, a change in resistance to current flowing through the magnetic element is produced. This change in resistance is a function of whether the memory element is in a stable Neel wall only state or a cross-tie Block line domain state, which are the two stable binary data states. The small magneto-resistance modulation produced during memory read-out of such devices gives rise to small signals which competes with both fixed pattern non-uniformities and substantial temporal noise.
U.S. Pat. No. Re. 30,087, owned by the assignee of the present invention, teaches a coherent sampled CMOS read-out circuit and signal processor coupled to a CCD shift register operated by a two-phase minority carrier transfer clock system. The signal processing taught by this prior art patent has also been termed correlated double sampling which enhances the signal to noise ratio of the output signal on the CCD devices. In U.S. Pat. No. 4,035,629, owned by the assignee of the present invention, an extended correlated double sampling signal processing technique is utilized for processing output signals from charge transfer devices in which the signal and a reference level sample are differenced in at least two stages to thereby correct the resultant output signal by reducing both temporal and spatial noise.